LONDON - Schools have closed their playgrounds and ambulance staff are reporting a surge in breathing problems as smog blankets parts of England for a second day.

Environmentalists criticized Prime Minister David Cameron after he said the haze -- a mix of local pollution, European emissions and Sahara sand -- was "a naturally occurring weather phenomenon."

As air pollution hit the top level of 10 in London and the southeast Thursday, Green Party European lawmaker Keith Taylor said Cameron's "flippant response to this invisible killer is utterly disgraceful."

Some schools kept children inside, while the London Ambulance Service said it was seeing a higher-than-normal number of calls from people with breathing difficulties, asthma and heart problems.

The World Health Organization says air pollution kills about 7 million people a year globally.